The present invention relates to a sheet delivery mechanism of an image forming apparatus, such as a printer or a copying machine, for discharging sheets of a printing medium carrying printed images to offset delivery positions.
When an image forming-apparatus for producing printed images on sheets of paper outputs multiple copies of printed sheets onto a sheet delivery tray provided outside a housing of the apparatus, for instance, it is usually difficult to discern boundaries between the individual copies and, therefore, a user needs to sort the individual copies upon completion of image forming operation. When producing two copies of a 10-page document by the image forming apparatus, for example, the user has to find out a boundary between the first and second copies, or the boundary between a last page of the first copy and a first page of the second copy, and separate the two copies from each other.
There are several known types of sheet delivery devices applicable to conventional image forming apparatuses for realizing efficient sorting operation. These sheet delivery devices are designed to selectively deliver multiple copies of printed sheets in different ways or at different (offset) locations by varying sheet delivery positions for easy sorting. Four specific approaches employed in these sheet delivery devices are as follows.
A first approach proposed in Japanese Laid-open Patent Publication No. 1999-199124, for example, is to feed printing paper of the same size in different orientations (portrait and landscape) for every other copy of a document and output printed sheets in the same orientations. Specifically, sheets for printing one copy are fed in such a way that a short side of each sheet goes first and sheets for printing next copy are fed in such a way that a long side of each sheet goes first. As the printed sheets are discharged in the same orientations, individual copies can be easily distinguished.
A second approach disclosed in Japanese Laid-open Patent Publication No. 2000-86056, for example, employs an offset tray. Although printed sheets are discharged from a fixed sheet output position, the offset tray is shifted (offset) to different positions so that the printed sheets are delivered to different locations for easy sorting.
A third approach proposed in Japanese Laid-open Patent Publication No. 1993-186121, for example, includes a pair of sheet output rollers individually mounted on two shafts and a differential gear mechanism provided between the two shafts. While the two sheet output rollers nip a printed sheet for discharging it, a difference is produced between rotating loads of the two sheet output rollers. The differential gear mechanism produces a difference in rotating speed between the two sheet output rollers so that multiple copies of printed sheets are output to different sheet delivery positions for easy sorting.
A fourth approach shown in Japanese Laid-open Patent Publication No. 1996-208091, for example, includes a driving roller assembly and pinch roller assemblies for discharging printed sheets. While a printed sheet to be discharged is nipped between the driving roller assembly and the pinch roller assemblies, the driving roller assembly is shifted in its axial direction. As the nipped sheet pulled by the driving roller assembly is also shifted in the axial direction of the driving roller assembly, the sheet delivery position of each sheet is varied to facilitate a sorting job.
An image forming apparatus recently developed is a so-called front access type as shown in FIG. 11, which is intended to achieve compactness in design. Referring to FIG. 11, the front-access-type image forming apparatus includes an image scanning section 160 located at an upper part, a sheet feed section 170 located at a lower part for feeding sheets of paper used for image forming, and an image forming section 180 disposed between the image scanning section 160 and the sheet feed section 170. The image scanning section 160, the image forming section 180 and the sheet feed section 170 are arranged generally in a U shape in cross section. A sheet delivery section 190 to which each sheet carrying a printed image is output is provided in a space between the image scanning section 160 and the sheet feed section 170. To meet an increasing demand for advanced features, this type of image forming apparatus incorporates a duplex (double-sided) image-forming function which is realized by a switchback sheet transport method instead of a normally used intermediate tray method. In the switchback sheet transport method, a sheet of paper is reversed by transferring the sheet in a direction opposite to an ordinary sheet transport direction through a sheet transport path S′ by means of a pair of sheet output rollers 191 immediately after an image has been formed on one side of the sheet.
For the front-access-type image forming apparatus, it is not desirable to employ the aforementioned first approach of Japanese Laid-open Patent Publication No. 1999-199124. This is because it is necessary to provide multiple sheet cassettes for each paper size to feed the printing paper in different orientations and this makes it difficult to achieve compactness of the apparatus. The aforementioned second approach of Japanese Laid-open Patent Publication No. 2000-86056 is not desirable for the front-access-type image forming apparatus either, because it is difficult to accommodate a movable offset tray in the limited space of the sheet delivery section 190. Accordingly, the aforementioned third and fourth approaches shown in Japanese Laid-open Patent Publication Nos. 1993-186121 and 1996-208091, respectively, seem to be suited to the front-access-type image forming apparatus, because arrangements of these approaches do not require a large space.
In the arrangements of Japanese Laid-open Patent Publication Nos. 1993-186121 and 1996-208091, however, sheets already discharged to different (offset) delivery positions on a sheet delivery tray, for example, might be displaced when another sheet is discharged onto the sheet delivery tray. This is because the sheet nipped by the output rollers, when discharged without a sheet output direction in which individual sheets are ejected from the output rollers being changed to a direction pointing away from a sheet delivery tray, goes into contact with at least one of the sheets already discharged onto the sheet delivery tray, and the sheet newly discharged could force, or push, one or more previously discharged sheets outward in the sheet transport direction. As a consequence, successively discharged multiple copies of the sheets might not be properly stacked at the intended delivery positions on the sheet delivery tray.